The in-situ ultrasonic measuring device for natural gas flux at the hydrocarbon seeps at the seafloor is a new technology of research and development at home and abroad in recent decades, and the research of this field is internationally thriving. The deep sea technology is regarded as a preferentially funded high-tech project of national sea field in the United States. In 1998, Professor Harry Roberts of Coastal Research Institute of Louisiana State University already designed and manufactured a natural gas flux measuring device, and successfully observed the natural gas seepage system at Bush Hill of Mexico Gulf. In 1999, Mexico Gulf Natural Gas Hydrate Research Union organized 15 colleges, 5 federal agencies and a few private companies, to research a real-time observation system for natural gas hydrate at the seafloor under the lead of the Mississippi University and carry out geochemical, microbial and seismic observation on Mississippi Canyon Block 118 of Mexico Gulf. Meanwhile, University of California, Santa Barbara observed global typical natural gas seepage and natural gas hydrate development zones of Hydrate Ridge, Mexico Gulf and the like by adopting the designed seafloor cold observation device. Hydrates developed in seepage systems (namely seepage type hydrates) have the characteristics of shallow burial, easiness in exploitation, high value and the like. The quantity of methane released to the sea water and the air through seepage of seafloor cold natural gas every year is very striking, and is more than 10 Tg (1012 g) every year as preliminary estimated. Methane is a strong greenhouse gas with a greenhouse effect approximately twenty times greater than that of an equal quantity of carbon dioxide, and may have an important impact on global climate change. Thus, it has an important economic value and scientific significance for in-situ and on-line measuring of natural gas flux at hydrocarbon seeps at the seafloor. At present, the research on the on-line measuring device for in-situ seepage flow of seafloor cold natural gas has been developed at home, and Guangzhou Institute of Geochemistry, Chinese Academy of Sciences has successfully researched an in-situ and on-line measuring devices for natural gas flux at hydrocarbon seeps at the seafloor, to fill the blank of China in this field. However, with the devices researched successfully, it is difficult to carry out long-term and multi-environment in-situ and on-line observation on the natural gas flux at hydrocarbon seeps at the seafloor due to the limitations of materials, components, power consumption and the like as well as poor working performance and stability of the devices.